The invention is in the field of ignition systems for fluid fuels such as gaseous and liquid fuels. In particular, the invention is an ignition system adapted to ignite gaseous fuel in furnaces that require either a pre-ignition purge or multiple ignition attempts.
The preferred embodiment of the instant invention is directed to a direct spark ignition system for pulse combustion furnaces. A pulse combustion furnace includes a combustion chamber having separate valved entrances for gaseous fuel and combustion air, and operates in the same fashion as a pulse-combustion jet engine, such as a "buzz bomb". Upon ignition of the fuel-air mixture, a positive pressure in the chamber is created, closing the inlet valves. Flow of combustion products through an exhaust pipe then creates a negative pressure in the chamber, drawing in additional fuel and air. This negative pressure also momentarily reverses the flow of departing combustion products, and hot combustion products igniting the fresh fuel-air mixture. Due to the nature of such a system, ignition may be difficult. A combustible mixture of fuel and air must exist within the spark path of the ignition means, or ignition fails, since there is no continuous flow of the fuel-air mixture once it enters the chamber. If ignition is unsuccessful, admitting additional fuel and air may change the mixture ratio to a noncombustible ratio, and may also establish a combustible and explosive mixture ratio in the exhaust pipe, with possible consequent damage. Therefore, the chamber and exhaust pipe are purged, by a forced draft of air, before and after ignition attempts.
Fuel and ignition controls for such furnaces, which accomplish this function, are known. One such prior control system utilizes a mechanical pressure switch to sense combustion chamber pressure as indicative of ignition, has a conventional oil furnace ignition transformer in an effort to provide high-energy ignition impulses for dependable ignition, and uses a motor-driven mechanical cam timer for controlling the fuel valve, the oil furnace ignition circuit, and a purge fan. Another known circuit utilizes discrete components, analogue in nature, and four relays. As will be apparent, such systems, having mechanical components, are prone to failure, and provide numerous modes of failure which cannot be rendered totally fail-safe, since the possibility exists that a fuel control valve may be maintained in an energized condition, when ignition has not been obtained.
The instant invention overcomes these and other deficiencies of the prior art.